Various types of roof vents for buildings have been devised in the past for various purposes including ventilation and air pressure equalization. Air pressure equalization is particularly important in building constructions having so called membrane type flat roofs in which a flexible, weather impervious membrane covers the roof. By virtue of its flexibility, the membrane covering can expand and contract in response temperature extremes and has therefore been used more frequently in recent years, particularly in commercial buildings. Wind blowing across the top of a membrane type roof can create areas of reduced air pressure above the roof which create forces that result in "ballooning", and in some cases, rupture of the membrane.
Pressure relieving roof vents are employed to equalize the pressure on the membrane and thereby avoid adverse affects on the membrane such as ballooning. Pressure relieving roof vents employed in the past typically comprise an upstanding tube mounted on the roof at a position overlying an opening in the membrane, thereby placing the area of the roof beneath the membrane in communication with open atmosphere. A one-way flapper valve mounted in the bottom of the tube allows the escape of air from within the roof beneath the membrane to the open atmosphere but prevents entry of air into the roof. A cap covering the top of the tube prevents atmospheric moisture and rain from entering the tube.
Pressure relieving roof vents of the type described above were less than completely satisfactory for several reasons. For example, moisture often formed on the interior sidewalls of the tube and/or on the value within the tube as a result of the condensation of moisture due to the warm air in the tube being exposed to the cool sidewalls of the tube when the atmospheric temperature is less than that of the air within tube. This moisture is drawn by gravity down the sidewall of the tube, accumulates on the flapper valve and gains entry into the roof when the valve opens. Entry of moisture through the membrane beneath the tube can result in serious damage and degradation of structural portions of the roof, such as wood or fiberous underlayment since the underlayment is directly exposed beneath the vent opening. Moreover, accumulated moisture at the bottom of the tube which overlies the flapper valve can freeze in cold weather, thereby resulting in seizure of the flapper valve.
Another problem associated with prior art roof vents involves the manner of mounting the vent on the roof. In the past, in order to temporarily secure the vent on the roof prior to applying the overlayment on the roof, it was necessary to tack the vent to the underlayment using nails or the like. In some installations this approach to the problem was undesirable since, once tacked down, it was impossible to readily adjust the position of the vent. Moreover, the use of nails for this purpose resulted in piercing of the membrane, thus making it necessary to assure that the nail apertures were subsequently sealed to prevent moisture entry.